Photographic print material is usually made by coating sensitized phtographic emulsion on to a waterproof opaque paper base or substrate (often referred to as "photobase"). The prepared paper base has to meet a rigorous specification not only optically and mechanically but also chemically. Thus, it should be inert to the chemistry of the photographic materials coated on it and used to develop the image, it should resist penetration of such chemicals (edge penetration beneath the waterproof coating at cut edges can be a problem), and it should provide adequate stiffness to the laminated material for acceptable mechanical processing and manual manipulation of the developed print.
The optical qualities of the photobase are, however, extremely important to photographic print sensitizer companies striving to offer the market a product of higher quality than their rivals. The photobase should be uniformly and densely opaque and constant in colour and of a specified degree of surface roughness to achieve the required gloss or matte finish. Clearly, such qualities are required in any substrate for an image.
Another optical quality of the image to which the substrate make a contribution is "image sharpness", which in this specification we have called "photodefinition". The nature of the surface of the substrate should reduce as little as possible the sharpness of the image actually achieved by the reaction of the sensitive material in the image-forming layer above the substrate to the pattern of energy incident on it.
Within the photobase industry it is understood that for highest photodefinition, the photobase should present as densely opaque a surface as possible, and that surface as close up behind the developed image as possible. Experience has shown that photodefinition suffers with any increase in distance between the opaque photobase and the photographic layer, as can occur for example with the use of a layer of gelatine as a means of bonding the photographic layer to the substrate. There is some explanation of this phenomenon in GB-A-1339045 of Fuji Photo Film Co. Ltd.
Conventionally photobase comprises a paper sheet (the "raw" photobase) coated on both sides with a waterproof coating, which in recent years is of polyethylene (except for some specialist products). The polyethylene is pigmented with a pigment which is white (except for specialist products) and which is usually titanium dioxide (TiO.sub.2). The usual method of coating is by extrusion of the coating on to the paper. The paper if identified with a "face side" and "wire side" (from Fourdrinier paper-making terminology) and it is the superior face side which when coated will receive the energy-sensitive imaging layer as a further coating.
There is a practical limit to the amount of TiO.sub.2, which can be incorporated in a polyethylene for extrusion. Above about 15 to 20 wt. % of TiO.sub.2, there is the prospect that the material will tend to accumulate on the lips of the extrusion die and thereby create a heterogeneous product. However, the polyethylene itself is quite capable of carrying much more TiO.sub.2 pigment. Indeed, it is supplied from pigment suppliers to photobase makers in the form of a "masterbatch", usually of 50 wt. % pigment 50% resin mixture.
In U.S. Pat. No. 4,263,080 of Whiting Jr. assigned to Ludlow Corporation there is described how a high opacity packaging material can be created by co-extruding a three-layer structure of which the middle layer has a very high loading of carbon black. By this means, it is stated, "severe rheological and sticking problems" are avoided, and there is "minimal build up of material on the die surfaces". The two outer co-extruded layers are of a thickness of 12 .mu.m.
The use of a co-extrusion method to make photobase has been proposed from time to time. See, for example, Fuji's GB-A-1339045 (mentioned above), Schoeller's GB-A-2061131 and Wiggins Teap's EP-A1-0 183 467. The last-mentioned disclosure is significant in that it discussed many different structures and is a relatively recent publication. It discloses the use of a combination of an upper layer of polycarbonate material, optionally pigmented, with a lower layer of pigmented polyethylene to achieve higher than expected levels of stiffness in the resulting photobase. It contains no mention of the consequences for image sharpness of moving from a mono-extruded to a co-extruded coating structure.